1. Technical Field
This invention relates to electrical isolation devices and particularly to an improved optocoupler device having a controllable output gain.
2. Background Art
It has become known that it is possible to design an isolation device for passing signals from an input to an output using optocoupling at the barrier between the input and output terminals. Such a device is the subject of U.S. Pat. No. 4,056,719 issued to S. G. Waaben on Nov. 1, 1977. The Waaben patent provides a solution to the problems of linearity of the output signal with respect to the input signal. Under the Waaben teaching a pair of light detecting diodes and a light emitting diode are used in a feedback arrangement such that the light output characteristics of the photodetector in the output circuit linearly track the input signal. Thus, as taught by Waaben, an optocoupler can be built that corrects for light output differences between LED's and photodiodes as well as corrects for temperature coefficient differences between devices.
However, a further problem exists primarily because of the inherent differences among the light transfer functions of different LED-photodetector pairs. This difference requires the gain of each device to be adjusted. Thus, the circuit which controls the bias of both the light-emitting and light-collecting diodes must be tuned to each circuit in order to insure highest precision signal gain. This tuning would typically be accomplished by varying the resistance values of the bias circuit. While such resistance matching is possible using potentiometers, such devices add undue expense to a production circuit.
Another approach would be to test the LED's and photodetectors and to match fixed resistors with each LED-photodetector pair prior to manufacturing. However, this solution is again extremely costly and viable only in limited production facilities.
Thus, a need exists in the art for a gain corrected optocoupler device which can be mass produced from available components without preselecting those components and without the need for externally gain correcting after manufacture.
A further need exists in the art for a transformerless isolation device which has a known gain output.
It is a general object of this invention to satisfy these and other needs by an optocoupled gain corrected device.